The first attempts to stabilise the frequency of a semiconductor laser diode began shortly after the successful CW operation of such laser diodes. The field received little interest until the end of the 1970's with the proposal of coherent-type optical fibre communications, which required highly coherent semiconductor lasers. Much of the research through the 1980's was aimed at coherent communications, however in the early 1990's, when the prospect of commercial WDM systems became a reality the emphasis shifted to frequency stabilisation for DWDM. This emphasis was fuelled by the introduction of proposed standardisation by the ITU in 1993, and recent efforts have been directed towards achieving practical standardised multichannel sources.
Locking the emission wavelengths of several lasers to different absorption lines in a species of gas received some attention. Thus an article by Tetu et al entitled, `Multiwavelength sources Using Laser Diodes Frequency-Locked to Atomic Resonances`, Journal of Lightwave Technology, Vol. 7, No. 10, October 1989, pp 1540-8, discloses the idea of locking the emission wavelengths of laser diodes to the wavelengths of selected atomic transitions in rubidium. That article explains that by adding foreign gas and altering the environment the authors were able to shift the peak wavelength of these resonances over a short region. Also by offset frequency locking of the lasers (i.e. use of a constant microwave beat-signal) the authors were able to achieve a small degree of control over the locked wavelength. This scheme gave channel separations of a few GHz or less, stabilised to a primary standard. However the spectral separation of the atomic lines is irregular and in the region of 0.811 .mu.m. An article by Sudo et al entitled, `Frequency-Stabilized DFB Laser Module Using 1.53159 .mu.m Absorption Line of C.sub.2 H.sub.2 `, IEEE Photonics Technology Letters, Vol. 1, No. 10, October 1989, pp 181-4, describes a similar scheme, but locking instead to the wavelengths of individual absorption lines of molecular acetylene. This has the advantage that these lines lie in the 1.5 .mu.m window and have a line separation in the region of 70 GHz. However, the line spacing is not perfectly constant: the separation converges at shorter wavelengths.
The devices described in the preceding paragraph are employed to generate a frequency comb filter, and then the emission frequencies of lasers are brought into alignment with the frequencies that register with the `teeth` of that comb. An alternative approach is to generate a comb of emission frequencies from a source emitting at a single frequency. Optical frequency comb generators are capable of producing an equally spaced comb of frequencies from a single source, over demonstrated ranges in excess of 2 THz, as for instance described by T Saitoh et al in the paper entitled, `Proposal of a Multiplex Optical frequency Comb Generation System`, IEEE Photonics Technology Letters, Vol. 8, No. 2, February 1996, pp 287-9. If the source of such a comb generator is primary stabilised, the possibility of locking an array of lasers to these lines exists. The spacing of the `teeth` of the comb is set by an electro-optic modulator, and is therefore restricted to the capabilities of the microwave oscillator circuit (realistically .about.-IOGHz). this makes the technique well suited for coherent systems, but less suitable for DWDM systems operating with a significantly larger channel separation, and moreover has the disadvantage of requiring a highly complicated set-up.
Frequency stabilisation techniques involving the use of scanning Fabry-Perot interferometers have also been described, for instance by T Mizuochi et al in the paper entitled, `Frequency Stabilized 622-Mb/s 16-Channel Optical FDM System and its Performance in 1.3/1.55-.mu.m Zero-Dispersion Fiber Transmission`, Journal of Lightwave Technology, Vol. 13, No. 10, October 1995, pp 1937-47. In theory such an instrument is capable of a settable channel spacing, and can also be configured to work with any primary reference. However they have the disadvantage of involving the use of expensive opto-mechanical scanning Fabry-Perot interferometers and a highly accurate timing mechanism. The emission frequency of an optical source may also be calibrated against that of a source emitting at a known frequency by the method described by M Guy et al in a paper presented at ICAPT '96, Montreal, Jul. 29-Aug. 1, 1996, entitled, `Fully-Packaged, Self-Calibrated, Absolute Optical Frequency Controller based on a Surface-Emitting Nonlinear Semiconductor Waveguide: Applications to Multifrequency Optical Communication`. This method uses a non-linear surface emitting device in a frequency meter configuration. The device emits the sum frequency of two orthogonally polarised beams at an angle proportional to the frequency difference between those beams. If one of the beams is a primary referenced laser, then the device operates as an absolute frequency meter. The angle is detected by free-space illumination of a detector array (CCD) that is analysed by a microcontroller to calculate the optical frequency. The stated accuracy of this device is sub-GHz at 1300 nm, but at 1550 nm is in the order of 4 GHz. This instrument is capable of calibrating itself to any two or more primary references in the 1550 nm window. Although the instrument is self-calibrating, the use of free-space optics is liable to make it less robust than instruments in which the optical propagation is waveguided throughout, and the components are relatively new technology and therefore expensive.
A much more simple and relatively inexpensive form of optical wavemeter is described by T Dimmick et al in the paper entitled, `Simple, inexpensive Wavemeter Implemented with a Fused Fiber Coupler`, Applied Optics, Vol. 36, No. 9, Mar. 20, 1997, pp 1898-1901. This instrument relies for its operation upon the wavelength sensitivity of the coupling provided by a fused fibre coupler. The input power applied to the input of this coupler is shared between its two outputs in ratio that is determined by the wavelength (frequency) of that input signal. The two outputs are coupled to a matched pair of photodetectors, and the electrical outputs of these two photodetectors are fed to the two inputs of a differential logarithmic amplifier. Initial calibration of the instrument was provided by first supplying it with light from a tuneable laser that itself has been previously calibrated in some unspecified manner. It is tacitly assumed that this calibration remains valid for the duration of the future use of the instrument.